The present invention is generally directed toward improvements in the television and video display arts. More particularly, the present invention is directed to a vertical video centering control system for providing a vertical centering function for a displayed video signal.
Traditionally, a television signal is displayed on a raster display with progressive horizontal line scanning in which successive horizontal lines of video are written on the screen from left to right, top to bottom. A vertical retrace function returns the scanning electron beam from the bottom right to the top left of the raster, typically in response to a vertical sync signal. That is, the horizontal line scanning is discontinued at some point in the lower right hand corner of the display screen and initiated in the upper left hand corner in response to the recognition of a vertical synchronization (sync) signal. The vertical position of the video on the display screen depends in part upon the relationship between the vertical sync signal and the video information to be displayed and also in part on the positioning of the display raster with respect to the overall display screen.
In N.T.S.C. standard television signals, vertical sync signals occur within a vertical blanking interval during which no video information is transmitted. In other applications, such as video monitors for computer related applications, a vertical sync signal is generally provided by the computer in accordance with the individual manufacturer's specifications and may or may not have video information immediately proceeding and following the actual vertical sync signal.
In standard television systems (N.T.S.C.) applications, vertical centering may not be considered critical, because the vertical blanking interval will typically extend for several horizontal lines on either side of the vertical sync signal so that no interference results in the displayed picture. However, with the advent of teletext and computer generated displays, use of greater portions of the available raster display is increasingly desirable. Unfortunately, in such applications the vertical timing information is typically provided by a vertical sync signal which is generally not standardized as to duration or exact time of initiation within any vertical field. Accordingly, it is desirable to provide a video monitor which is compatible with various systems and allows for the video display to be vertically centered when desired.
The positioning of a video display field on a video tube is also controlled in part by the video tube deflection yoke which controls the direction and movement of the raster tracing, scanning electron beam. It is well known in the art that some measure of vertical centering may be provided by introducing a D.C. offset into the deflection yoke to alter its reference or starting point and correspondingly shift the entire display raster on the display screen. However, this method traditionally creates undesirable effects on the linearity of the system and may be unsafe when accessible to the consumer.
An alternative method for providing some measure of vertical video centering is to shift the vertical sync signal with respect to the video information by delaying the sync signal. However, this allows solely for a video shift upward on the raster, because the vertical sync signal may be shifted in only one direction by delaying it.
Another method for shifting the sync signal with respect to the video signal is to delay each sync signal for approximately the duration for one vertical field interval so that each vertical sync pulse will be used to synchronize the succeeding field of video information. In such a system, the video may be moved both up and down on the display raster by delaying each sync pulse somewhat more or less than the typical duration of an entire vertical display field. However, undesirable effects may frequently be encountered when raster scanning is timed to the preceding field sync pulse while displaying video from a current vertical field.
Yet another method for providing some measure of vertical video centering is to delay the video information itself with respect to the sync pulse with the use of a long delay line. However, this is quite difficult and costly.
Accordingly, it is desirable to provide a system for allowing relatively easy vertical video centering, both up and down on the video display raster, without creating undesirable side effects on product safety, picture linearity, video bandwidth or video definition. Therefore, the principal object of the present invention is to provide a vertical video centering control system that generally overcomes the deficiencies of the prior art.
It is a futher object of the present invention to provide a vertical video centering control system which will allow the video information to be moved both up and down on the video display raster.
Still a further object of the present invention is to provide a phase locked loop system to shift existing sync signals within a vertical field interval to accomplish vertical centering.